Medical instruments must be cleaned and sterilized before every use. Sterilization is performed as a rule by autoclaving, that is, in a pure steam atmosphere at high pressure and at temperatures well over 100 degrees Celsius. Penetration of steam into an endoscope or other optical medical instrument would have disadvantageous consequences for the instrument's optical properties.
As a rule, therefore, optical medical instruments comprise hermetically sealed and insulated apertures through which light must pass in and out, for example on the distal and proximal ends of an endoscope, and which are closed off by window components made of a transparent material. Many window components are made of sapphire, which is characterized by great hardness and chemical resistance. To form a hermetically sealed connection, the edge of a sapphire window can be joined with the housing by soldering. Sapphire, however, is (single-axis) streaming-birefringent and therefore is not unconditionally suited for diagnostic methods that use polarized light.
Glasses are not birefringent but are frequently disturbed by thermally induced mechanical stresses immediately upon cooling after the soldering procedure. An additional problem consists in the fact that a few conventional glass solders are not biocompatible with advantageous mechanical properties relating to manufacturing. Therefore these glass solders cannot automatically be used for medical instruments that come into contact with the human body.